1. Technical Field
The present invention relates generally to interconnect semiconductor structures, and more particularly, to a method of detecting devices lacking at least a portion of a protective hardmask covering an underlying material.
2. Related Art
A hardmask can be deposited over a dielectric layer during the formation of damascene structures. The hardmask acts as an etch stop layer and/or a chemical-mechanical polish (CMP) stop layer during the formation of conductive features within the dielectric layer. The hardmask also acts as a protective barrier preventing ambient moisture from diffusing into the dielectric layer. Moisture that diffuses into the dielectric layer can eventually attack the conductive features within the dielectric layer and lead to catastrophic device failure. Clearly, it is undesirable to have regions of the dielectric layer that do not have the protective hardmask thereover.
Unfortunately, portions of the hardmask can be inadvertently removed during the chemical-mechanical polishing (CMP) of the conductive features within the dielectric layer. This is particularly problematic in regions of high metal density. Regions are considered high metal density regions when the ratio of the conductive features to the spaces between the conductive features is greater than 50%. For example, a grouping of conductive wires having a width of 1.8 microns spaced about 0.2 microns apart would be characterized as having 90% metal density. This type of region would be especially susceptible to hardmask removal. The removal of the hardmask occurs because the CMP polishes through the conductive material, e.g., copper, much faster than the hardmask. Since the conductive features are so close together the small islands of hardmask extending above the plane of the conductive features are often removed during the CMP polish.
The problem arises in detecting those devices having regions that are lacking the protective hardmask before the device is implemented, or shipped to the end user. Therefore, there is a need in the industry for a method of detecting devices having regions lacking a protective hardmask.